1. Field of the Invention
This invention relates to a developing powder used for electrophotographic methods, electrostatic recording methods, magnetic recording methods, and the like.
2. Description of the Prior Art
Heretofore, various electrophotographic methods have been known, for example, those as disclosed in U.S. Pat. Nos. 2,297,691, 3,666,363, 4,071,361 and the like. In general, electrophotographic methods comprise utilizing a photoconductive material, forming electric latent images on a photosensitive member by various means, developing the resulting latent images with a developing powder (hereinafter referred to as "toner"), if desired, transferring the toner images to a receiving member such as paper, and then fixing the toner images by heat, pressure or a solvent vapour to produce a copy. Where there is a step of transferring toner images, there is usually provided a step of removing the remaining toner on the photosensitive member.
A wide variety of techniques are known for visualizing electric latent images with a toner such as magnetic brush (e.g. U.S. Pat. No. 2,874,063), cascading developing (e.g. U.S. Pat. No. 2,618,552), powder cloud method (e.g. U.S. Pat. No. 2,221,776), conductive magnetic toner method (e.g. U.S. Pat. No. 3,909,258) and the like.
As the toners suitable for these developing methods, there have been used finely divided particles of natural or synthetic resins in which dye or pigment is dispersed. For example, finely divided particles of 1-30.mu. in size of a resinous binder such as polystyrene and the like containing a dispersed colorant can be mentioned. As the magnetic toner, a toner containing particles of a magnetic material such as magnetite is used. As a so-called two-component developing agent, the toner is usually used together with carrier particles such as glass beads, iron powders and the like. These toners are required to have various physical or chemical characteristics, but most of prior art toners suffer from some of the following drawbacks.
Most of the toners which easily melt by heat often cake or coagulate during storage or in copying machines. Most of toners are deteriorated with respect to triboelectric property and flowability depending upon the ambient temperature change. Further, when toner particles and carrier particles are continuously used for repeated copying, these particles collide each other and contact the surface of the photosensitive plate resulting in mutual degradation of the toners, carrier particles and photosensitive plate. Thus, the resulting image density changes and are not uniform, the fog density increases or quality of the copy is lowered. Moreover, in the case of most of toners, when the amount of toner attached to a photosensitive plate bearing latent images is increased so as to enhance the developed image density, the fog density usually increases.
Among those undesirable phonomena, there is included a phenomenon which occurs due to brittleness of the toner particles, that is, a phenomenon concerning the life of the developer. In the developing vessel the toner particles are brought in contact with the latent images and during said process the toner particles are subjected to load by blades, transferring system, other toner particles, carrier particles and the like. Toner particles are crushed by this load, and adhere to carrier particles and remain in the developing vessel resulting in deterioration of the developer. In order to avoid such deterioration, polymers of high molecular weight may be used, but the fixation temperature is elevated upon thermal fixation and this disadvantageously results in a large consumption of thermal energy.
Furthermore, in order to eliminate the above metnioned drawback, it is proposed to add a small amount of a plasticizer to the toner, but such addition adversely affects the free flowability of the toner and causes contamination of carriers, and this proposal can not successfully solve the problem.
These problems are common to both one-component development and two-component development. In general, one-component developing system is more advantageous than two-component developing system, but one-component developing system has its inherent disadvantages. That is, in the developing process using a one-component magnetic toner, the toner particles do not move about on the toner holding member to a great extent because the agitation operation is so little. Therefore, when a small original is repeatedly copied, magnetic toner particles present at a site of the toner holding member where no development is carried out (e.g. the site is outside of the portion corresponding to the area of the small original) simply rotate together with the toner holding member to which said toner particles attach, and new magnetic toner particles are hardly supplied to said magnetic toner particles.
For example, when, after producing a great many sheets of coapy of a A-4 size original, a B-4 size original having a wider area than a A-4 size original is copies, a portion of the resulting image developed with toner particles existing at a portion which is not used for developing the A-4 size image has a lower image density than the other portion of the resulting image developed with toner particles existing at a portion which is used for developing the A-4 size image. The reason why such lower density is obtained is not yet clear, but it is considered that in general, one-component magnetic toner particles hardly move about and the one-component magnetic toner particles attached to a portion of a toner holding member which is not used for development are repeatedly rotated without being replaced by new toner particles, and this appears to be the cause.